According to the flight instructor, he and the student pilot had practiced various maneuvers during the training flight and were returning to the airport. The airplane was about 2,500 feet above ground level and within gliding distance of the airport when the flight instructor turned the fuel selector to the "OFF" position so the student pilot could practice engine failure procedures. (Although no regulations prohibit turning the fuel selector to the off position while practicing engine failure procedures, the practice results in an actual, not a simulated, loss of engine power, and is therefore inherently more risky than a reduction in engine power to idle, especially when initiated at a low altitude.) The student pilot trimmed the airplane for best glide speed, initiated a turn toward the airport, and reviewed the pertinent emergency procedure guidance in the pilot operating handbook. The student pilot could not locate the airport until the flight instructor directed him. When the student pilot was unable to restart the engine, the flight instructor took control of the airplane and tried to restart the engine. The flight instructor’s efforts to restart the engine were also unsuccessful, and, believing it was too risky to try to reach the airport, the flight instructor selected the best available field for a forced landing. During the landing, the nose landing gear impacted the ground and the firewall was substantially damaged. The student pilot, who was also a mechanic, reported that there were no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal engine restart and operation.